This invention is for a transducer responsive control system which is interposed between a transducer and the apparatus which the transducer monitors and controls. A transducer as herein referred to is an instrument which responds to the variations in liquid level, pressure, or temperature in an apparatus wherein liquid level, pressure or heat are converted into a continuous electric current that fluctuates either directly or inversely with the changes in liquid level or pressure within a selected range in the monitored apparatus. In most apparatus to which the transducer responds, a signal reflecting minor and relatively insignificant variations in liquid level or pressure is generated causing quite frequent and often almost constant changes and reversals up or down in the throttling of a supply valve or the operation of a motor, etc., resulting in frequent service calls and earlier replacement than should be necessary.
The present invention is for a system or apparatus whereby the output signal from the transducer is converted, without interruption to the continuity of the signal current from the transducer, into a series of steps of preferably, but not necessarily, equal magnitude at predetermined stages, whether arrived at cumulatively by a succession of minor variations in the same direction or by a continuous rise or fall from one stage to another in the parameters to which the transducer responds, but eliminating minor or unimportant fluctuations between stages.
Since the present invention was initially conceived for use in connection with the control, or throttling of a supply valve to a water storage tower in a municipal or like water supply system, it may be easily followed by reference to this use where it has special application. In such application the transducer signal is reflected in the throttling or adjustment of a valve in steps or stages between fully open and fully closed positions with provision also for programming the degree of opening or closing of the valve at each step or stage to compensate for the fact that a 10.degree. opening of a valve, for instance from fully closed to the 10.degree. open position, may effect a much greater increase in volume of flow than, for example, the same extent of opening between 70.degree. to 80.degree. of opening.
In municipal and like water distribution systems, it is quite common to have a water tower of a height of perhaps 40 meters from which water is distributed through mains and individual service lines to the residents, businesses, shops, etc. in the community. The demands for water change from hour to hour, day to day, and season to season. The supply of water to the tower to meet these demands is provided by a supply main through an electrically controlled throttling valve which is responsive to the rise and fall in the level of water in the tank.
Assuming a tank is 40 meters high, the maximum depth of water would be 39 meters in order to leave an air space of one meter. This being the case, the supply valve must be closed when the 39 meter depth is reached. It may be assumed in this system that as the water level drops from this maximum level to 32 meters or below, the valve will be fully open. Ideally, the inflow through the supply valve and the outflow from the tank should maintain the level of water in the tank as close as possible to some intermediate level as established by the demand then existing.
Heretofore, a transducer resonding to the level of water in the tank as the water level varied to any measurable extent, up or down, throttled the supply valve to effect an opening or closing of the supply valve through a servo amplifier which signaled the degree of valve opening as the water level in the tank decreased, until the supply valve ultimately might reach a fully open condition when the water level dropped to the 32 meter level or below. Conversely, if the demand for water from the tower decreases and the level then rises, the valve will progressively close, reaching a fully closed position at the 39-meter level.
Since the water level in the tower is practically never static but is continuously rising or falling, the transducer transmits a correspondingly continuously fluctuating signal to the servo amplifier which, in turn, continuously commands the valve to adjust one way or another. This results in frequent demands for servicing and shortens the useful life of the equipment.
Moreover, with the present system the valve opens or closes as though each degree of opening resulted in an increase or decrease in flow, as did any other degree where, as pointed out above, one degree of change when the valve is nearly closed affects the flow to a greater or lesser extent than one degree of change as the valve approaches a fully open position. That is, the presently used apparatus assumes a linear increase in variation whereas, in fact, the flow through the valve as it opens or closes must be graphically represented as a curve, dropping away steeply from the closed position and then curving laterally near the full open position. The successive energizing of these relays or their successive de-energizing transmits an impulse to the servo amplifier to increase or decrease, as the case may be, the degree of opening or closing of the throttling or supply valve.
With the present invention this variable transducer signal is not transmitted to the servo amplifier but is transmitted instead to a unit between the transducer and servo amplifier that produces from the frequently varying transducer signal a stepped signal in which each step is of predetermined magnitude, up or down, and variations in the transducer signal that occur between steps are never transmitted to the servo amplifier. Each signal to the servo amplifier, up or down, will be ordinarily but not necessarily for a step of equal height to every other step, typically one meter below a selected height, except at the top and bottom of the range where the valve is fully closed at the top of the range and is fully open at the bottom.
However, since the change in rate of flow through a valve will not vary equally with each degree of valve opening or closing, a rate of flow with the valve open 20% from a fully closed position may differ substantially from the rate of flow with a change in 20% opening or closing with the valve already 50% or 60% or 70% open. This invention further provides a programmable opening or closing of the valve to different degrees at different levels where the magnitude of the steps, i.e. for example one meter, will be equal at each step but the degree to which the valve is opened or closed may be adjusted so that the rate of flow of water at each level may be equalized to that at the others, or otherwise programmed, perhaps to give a greater rate of flow in the intermediate levels to keep closer pace with the demand. If the level of the water falls below the lowermost level of the scale at 32 meters in the assumed example, the supply valve will remain at the fully open position at the bottom of the scale. For simplicity of explanation we assume herein steps of equal magnitude; but as will be readily understood by those skilled in the art, this is not necessarily the case. For example, where ice formation conditions may be a problem, the steps may be unequal but the circuitry, in principal, will not change.
The stepping sequence is controlled by a comparator which energizes or de-energizes a succession of relays which transmit a stepped signal to a servo amplifier that commands the opening or closing of the supply valve, the extent or degree of opening or closing of the valve being regulated by a potentiometer or equivalent signal magnitude adjusting means between each of the respective relay generated signals to the servo amplifier, as hereinafter described in detail.
Important to the operation of the invention, no signal may be transmitted to any relay to increase the opening of the valve until the previous relay in the series has been first energized and no relay, therefore, can operate out of sequence. When a closing sequence of the valve is directed, which means deenergizing the relays in succession, no relay can be de-energized until the preceding relay has been first de-energized. This assumes that no opening or closing of the valve can take place at any time but only in steps from at least one stage to the next. We refer to this part of the apparatus as the "stepper".
Finally, the inclusion of a light emitting diode (LED) in each relay circuit gives a visual indication of that relay circuit which, at the time it is lighted, gives an approximate indication of the liquid level in the tank. If it is full, one LED is lighted; if the valve is full open, another LED is lighted, and the intervening diodes in this seqence indicate the level to which water is being supplied. A two-color red and green LED is desirably included in the system to indicate whether at any particular time the system is responding to a rising or falling level in the storage tank.